A Study of an Investigational Drug, Cemdisiran (ALN-CC5), in Patients With Atypical Hemolytic Uremic Syndrome

Inclusion Criteria: 1. Willing to provide written informed consent and to comply with the study requirements 2. Age 18 years or older 3. Clinical diagnosis of primary aHUS 4. Clinical thrombotic microangiopathy (TMA) activity 5. Women of child-bearing potential must have a negative pregnancy test, cannot be breast feeding, and must be willing to use a highly effective method of contraception 6. Previously vaccinated with meningococcal group ACWY conjugate vaccine and meningococcal group B vaccine or willingness to receive these vaccinations 7. ADAMTS13 \>10% or other proven aHUS-associated mutation Exclusion Criteria: 1. Clinically significant abnormal laboratory results 2. Positive Shiga toxin producing Escherichia coli test at Screening 3. Suspected secondary aHUS, in the opinion of the Investigator (unless there is a documented aHUS-associated genetic mutation) 4. Positive direct Coombs test 5. Patients who have received hemodialysis for \>3 months 6. Bone marrow transplant recipients 7. Organ transplant recipients, except for kidney transplant recipients with primary aHUS (confirmed by known genetic mutation and kidney biopsy) 8. Known history or evidence of systemic lupus erythematosus or antiphospholipid antibody syndrome 9. History of multiple drug allergies or history of allergic reaction to an oligonucleotide or GalNAc 10. Malignancy (except for non-melanoma skin cancers, cervical in-situ carcinoma, breast ductal carcinoma in situ, or stage 1 prostate carcinoma) within the last 5 years 11. Patients with a poor prognosis that is expected to limit their life expectancy to less than 3 months, in the opinion of the Investigator